Left-click in view, then use your arrow keys to move around in 3D space (with control-key for up and down). Hit the 'f' key to reset to front view, 'b' for back, 'l' for left, 'r' for right. Click on a senator to zoom. Move your mouse cursor over a senator to see their name and their details.
In the front view, the senators towards the upper left are Democrats; the senators towards the lower right are Republicans.

The senators' locations in 3D space are determined by their votes in the 111th Congress as reported by http://www.govtrack.us/. A total of 696 votes were cast, though not every senator cast every vote. Senators who voted the same on many bills appear nearby each other; senators who voted differently appear further apart. It's self-organizing.
The points were generated from the raw data by a technique called multi-dimensional scaling via relaxation.